1. Field of the Invention
The present invention relates generally to fill tubes for transferring molten metal into a casting mold and, more particularly, to a compliant fill tube assembly that maintains a substantially leak-proof seal between the fill tube and the casting mold while accommodating dimensional variations, due to, for example, thermal changes, tolerance ranges, component degradation and assembly errors. The invention also relates to a fill tube for the foregoing fill tube assembly and to a method of use.
2. Background Information
To avoid commonly known problems associated with casting molten metals by pouring the melt into a mold, for example, by utilizing the assistance of gravity, molten metals, such as molten aluminum, are typically bottom-pressure cast also known as reverse casting or anti-gravity casting. One such casting technique is commonly known in the art as vacuum-riserless, pressure-riserless casting, wherein molten metal travels upward from a melting furnace or bath, through a fill tube and into a mold cavity. At the top of the mold, a vacuum is pulled to evacuate air within the mold. Pressure is then applied to the molten metal in the melting furnace, thereby forcing it up, through the fill tube and into the evacuated mold. After filling the mold, metal in the tube runs back down into the melting furnace, thereby avoiding solidification of metal within the fill tube and problems, such as, contamination and metallurgical defects, associated therewith.
Effective vacuum-riserless, pressure-riserless casting relies on an air-tight seal between the fill tube and the casting mold throughout the duration of the casting process. The fill tube in such casting systems can be made from a variety of materials, such as, for example, titanium and ceramic materials or any other material which will maintain its stability, structure and other properties when in contact with molten metal. It is well known in the art that ceramic materials exhibit good material properties in compression, but respond quite poorly to tensile stresses. Accordingly, there has been a longstanding problem in the art of reverse casting of failing or fracturing fill tubes and the inability to maintain a continuous air-tight seat between the fill tube and the casting mold. Many of these problems are associated with, for example, over-tightening the fill tube and thus breaking it while attempting to form a sufficiently tight seal. Another frequent source of fill tube assembly malfunction stems from very tight fill tube assembly tolerances which cannot accommodate dimensional variations or assembly errors. Such dimensional variations can cause uneven loading and sealing problems at the fill tube to casting mold interface, permitting the infiltration of air around the seal into the mold which can result in casting problems such as, for example, fill tube failure, leaking fill tube assemblies, production of scrap castings and downtime of the casting process all of which increase the costs of the cast product.
Dimensional variation may result from, for example: thermal expansion and contraction of fill tube assembly components resulting from temperature variations during the casting process; design or fabrication errors or tolerance variations in the fabricated fill tube assembly components; and fill tube assembly component degradation. Fill tube assembly errors may include, for example: bolt tightening sequencing; overloading of assembly components; and alignment of assembly components.
Known prior art fill tubes and fill tube assemblies typically employ a very rigid, tight tolerance fill tube to casting mold interfaces and produce unacceptable tensile stress with respect to both the magnitude of stress and the size of area exposed to such stress. Additionally, many known fill tube and fill tube assemblies require a clamping assembly design that requires very tight tolerance requirements in the production of the fill tube, which increases costs of production. Other known clamping assembly designs have little or no tolerance to assembly errors and employ a fill tube design with significant variations in cross section that can produce undesirable stress risers. See e.g., U.S. Pat. No. 5,919,392 (discussing the shortcomings of several known, patented, fill tube and fill tube assembly designs). Such fill tube assemblies do not provide any compliance to compensate for or accommodate the foregoing dimensional tolerances of the fabricated components, dimensional changes due to thermal changes over time or assembly errors. Moreover, typical fill tube assemblies are heavy and not installation friendly.
There is, therefore, a need to provide a fill tube, fill tube assembly and method of use thereof that can accommodate dimensional variations occurring during assembly of the fill tube assembly as well as dimensional variations due to thermal changes of the fill tube assembly components occurring during casting operations.
There is a further need for such a fill tube, fill tube assembly and method of use thereof that can provide and maintain a substantially air-tight seal at the fill tube to casting mold interface when employed, for example, in casting operations employing a vacuum, such as, for example, vacuum-riserless, pressure-riserless casting.
There is, therefore, room for improvement in the art of fill tubes, fill tube assemblies and methods of use thereof.